This invention relates to a method of adjusting the lip opening of an extrusion die for stratiform material, particularly for extrusion dies of the type that mainly consists of, on the one side two lips and the like, forming the lip opening, and on the other hand, a number of control units put across the width of the lip opening, which all consist of one or more bar shaped elements, expansive by heat, respectively shrinkable by cooling which act upon at least one the lips in order to displace them and to change the lip opening. These dies are appropriate for the extrusion of film and coating as well as for the extrusion of sheet and the like.
An extrusion die of said type is known from the U.S. Pat. No. 3,940,221. This known extrusion die mainly consists of a die block in which an oblong opening is provided through which a more or less liquid material may be pressed by extrusion to form a layer, whereby the opening is limited on one side by flexible lip which may be moved by expansive, respectively shrinkable, bar shaped elements in order to modify the opening width. The heating, respectively the cooling, of the bar shaped elements is hereby done by means of heat-conducting blocks which are provided individually for each bar shaped element, and which, parallel to the bar shaped elements, each consist of a heating element and a cooling channel.
Although such an arrangement theoretically enables a proper control of the lip opening, it shows the practical difficulty of distributing the cooling medium over a large number of cooling channels in such a manner that approximately the same amount of cooling medium is delivered to each cooling channel. So it is rather difficult to provide a uniform cooling effect in all the bar shaped elements.
Moreover, such an arrangement shows the disadvantage that, when one cooling channel fails, the corresponding bar shaped element doesn""t receive any more cooling at all. The increased activity of this element, originated by this condition, also influences the adjacent bar shaped elements, which eventually results in a total deranging of the system.
At the same time, with the known arrangement, the cooling channels are located between the bar shaped elements and the heating elements. From this the disadvantage arises that a considerable supply of energy to the heating elements is required to provide the bar shaped elements with an encreased heat supply, opposite to the cooling.
Further, this known extrusion die also shows the disadvantage that the control of the lip opening is very susceptible to the changes of the ambient parameters. This is, amongst other things, due to the fact that the bar shaped elements, in order to fasten them, are in contact with the die block over a comparatively large distance, so that a considerable heat bridge is existing between the die block and these elements.
Thus, the present invention also relates to an extrusion die for the extrusion of film, coating, sheet and the like, showing none of the said disadvantages nor other ones.
The present extrusion die has the advantage that in the surroundings of the bar shaped elements a considerable uniformity of the temperature is reached, whereby also various temperature influences, by air currents and head bridges, are leveled out. By this, one obtains also a uniform radiation of heat to drums possibly positioned in the surroundings of thee die, so that the disadvantage of the so called hot spots is avoided. Another advantage of the extrusion die according to the present invention consists in the possibility of operating with lower mean temperatures of the bar shaped elements, so that one obtains that the temperatures of the system may be adjusted in such a manner that the time constant of the cooling is almost equal to the mean time constant of the heating of the bar shaped elements. As it is known, this is a primary requirement to enable a proper control. By an operation at lower temperatures a smaller difference between the temperature of the device and the ambient air is obtained, whereby the influence of the surroundings on the system is considerably reduced. The extrusion die also shows the advantage that the bar shaped elements are very well shielded against external influences, whereby it is obtained that, when the control system and/or the power supply fail the extrusion die easily may continue to operate by means of manual control.
The extrusion die according to the invention shows also the advantage that, thanks to the method applied to control the lip opening and more specially to cool the bar shaped elements, the bar shaped elements keep receiving a sufficient cooling when one cooling channel fails.
The said advantages are mainly obtained by the application of a method for the control of the lip opening of an extrusion die, whereby the extrusion die is of the type that mainly consists of, on the one side, two lips forming the lip opening, and on the other hand, a number of control units put across the width of the lip opening which all consist of at least one bar shaped element, expansive by heating, respectively shrinkable by cooling which act upon at least one of the lips in order to displace them and to change the lip opening, and whereby the method for the control mainly consists of the combination of, on the one side, the control of the heating of the bar shaped elements by each control unit in function of the local thickness checks which are carried out across the width of the formed layer of material, and on the other hand, the common cooling of several control units by forcing cooling medium mainly transversely along several bar shaped elements.
According to a preferred method according to the invention also the temperature of the common cooling medium at the supply thereof will be controlled in combination with the said common cooling.